1. Field of the Invention
The invention relates to an industrial truck with at least one battery. The invention likewise relates to a method for operating an industrial truck with at least one battery.
2. Technical Considerations
Such industrial trucks are used for the in-house transportation of goods, preferably when using electrical traction and lifting drives where it is necessary to avoid damaging exhaust gas emissions, in particular in internal spaces. In comparison with vehicles driven by an internal combustion engine, however, reduced use times need to be accepted since recharging or replacement of the battery is substantially more complex and more time-intensive than a tank-filling operation. For this reason, frequent charging or replacement operations should be avoided, which can be achieved in particular by optimized utilization of the energy supply stored in the battery. One of the preconditions for this is knowledge of the present state of charge of the battery which is as precise as possible since, when the available charge is underestimated, charging operations which are too frequent occur, while, when the state of charge is overestimated, the vehicle may fail unexpectedly, which means a substantially more serious restriction of the use time and, owing to the deep discharge, results in a shortened life of the battery.
In order to determine the energy supply stored in the battery, various methods are known. One of these consists in determining the state of charge by measuring the open terminal voltage of the battery. These two variables are approximately proportional to one another when the battery is off load. When an industrial truck is operated, however, various factors occur which may result in a falsification of the result: since, on load, the battery voltage is reduced, the energy content available is underestimated in the case of a state-of-charge display based on the voltage, which results in an unnecessarily premature charging operation.
A further problem in all methods which are based on voltage measurements is the occurrence of the so-called diffusion overvoltage, in which, owing to the loading of the battery, a concentration gradient occurs in the electrolyte, which results in a reduction in the terminal voltage. This phenomenon occurs particularly markedly in the case of the lead-acid batteries usually used in industrial trucks. As a diffusion-controlled process, the level of the diffusion overvoltage changes in the event of a change in the power drawn with a considerable time delay, with the result that, in the case of state-of-charge displays which are based on the measured terminal voltage, the display of the state of charge lags behind the actual available capacity. If this is not taken into account, the energy supply on load is overestimated and this may result in a damaging deep discharge of the battery. When there is no load, the distribution is brought back to normal again after a certain rest time, and the full capacity is available again. A simple state-of-charge display based on the battery voltage is therefore associated with many uncertainties and is therefore only suitable in certain circumstances for use in an industrial truck.
Greater accuracy in terms of the energy supply stored in the battery is obtained with methods in which, owing to the determination of the incoming or outgoing currents and integration of these measured values, the charge content of the battery is determined. In the event of a battery replacement, as is often carried out in practice in the case of industrial trucks, since the time required for this is substantially shorter than that required for charging the battery, however, the amount of usable charge contained in the new battery is not known. This disadvantage can be circumvented in part by battery data being stored on the battery or in a control device, but this is relatively complex.
Methods are also known which carry out both current and voltage measurements in order to determine the state of charge of the battery. For example, the capacity of the battery when inserted into the vehicle is determined from the measured open-circuit voltage and, when power is drawn, by integrating the measured current the drawn charge and therefore also the remaining residual capacity are determined. It is also known from U.S. Pat. No. 4,333,149 to calculate a so-called dynamic internal resistance, which has a relationship with respect to the state of charge, from the voltage drop when the battery is on load and from the drawn current. However, this method requires precise knowledge on the battery and does not take sufficient account of many boundary conditions, such as line losses between electrodes and measuring devices, for example. In order to obtain such knowledge, lengthy learning phases are required for each battery. In the case of an industrial truck which is often operated with replaceable batteries, this complex adaptation also needs to be carried out for each battery, and the apparatus needs to be operated with the respectively appropriate battery parameters. In the case of these methods, too, owing to the dependence on the measured voltage values, the occurrence of the diffusion overvoltage may result in a falsification of the information on the state of charge.
The methods or apparatuses known for industrial trucks with at least one battery in accordance with the known art for calculating and displaying the electrical energy which can be drawn from the battery are therefore either very complex or only produce unreliable results, which either systematically over- or underestimate the true state, depending on the method, or else deviate unpredictably depending on the discharge conditions.
Disadvantageous factors here, in particular in addition to the complex design, are the slow response to the severely fluctuating operating conditions of an industrial truck and the associated influences on the available battery capacity as well as the necessity to determine precise information on the battery used using complex methods. That is to say, if incorrect battery parameters are used, in most methods this may result in serious erroneous estimations of the state of charge with the known negative consequences.